To significantly improve the security of homes and business against break-ins by unauthorized persons, double-cylinder lock deadbolts are widely used Insurance companies and police highly recommend the use of such locks as a deterrent to burglaries. The greatest security is obtained from double-cylinder locks, which are key operated from both the inside and outside of the door in which they are placed. When the bolt is engaged (that is, when the door is locked) and both keys are removed, a significant deterrent against break-ins is provided.
When a key in the inner cylinder is used, however, a safety hazard is presented to the occupant of the dwelling if the bolt of the lock is engaged and the key is removed from the cylinder. In the event of a fire, or if for any other reason the occupants need to leave the room or building quickly, the time required in a panic situation to locate a key to release the lock, could result in a disaster.
Consequently, in some areas laws have been enacted which prevent locksmiths from installing double-cylinder key operated deadbolts, because of this fire or panic danger. The inside of a deadbolt lock typically then is provided with a permanently affixed handle to permit its operation While the level of protection against unauthorized use of the door is reduced when a handle permanently affixed to the inner operation of the deadbolt is used, the safety problem is alleviated.
Many dwellings, particularly homes, already exist, which have a key operated cylinder on the inside of a double-cylinder deadbolt. It is expensive to replace such a lock with a non-key operated lock, and many cases it is necessary to replace the entire locking mechanism if the key operated inner cylinder is no longer desired.
Some efforts have been made to retain the inside key in a double-cylinder deadbolt lock, whenever the occupants of the dwelling are located inside the dwelling, to prevent it from being missing in the event of an emergency such as a fire when the persons inside the building desire to leave quickly. This has been accomplished by specially designed lock mechanisms, for example the lock mechanism shown in the Oliver U.S. Pat. No. 4,315,420. That lock mechanism is designed with a mechanical latch in it to engage with a notch on a specially designed inside key, to retain the inside key at all times in the lock unless removal of the key is desired. Such removal is effected by operation of a cam on the key plug on the outside locking cylinder. A problem with the lock mechanism of this patent, however, is that it is a specially designed lock mechanism. It is not a standard double-cylinder deadbolt lock.
Another patent, which uses a non-standard lock mechanism to trap the key on the inside lock, is disclosed in the patent to Allemann U.S. Pat. No. 4,109,496. This mechanism is designed so that whenever the deadbolt is extended to a locked position, the inside key cannot be removed from the lock. Consequently, the lock provides the desired safety features; but, again, a non-standard specially designed double-action lock must be used.
Two older patents, to Helms U.S. Pat.No. 2,441,067 and Kindler U.S. Pat. No. 2,528,757, disclose a rotating arm for captivating old-fashioned keys in the keyhole of a deadbolt lock. This prevents removal of the key. The lever arm extends close to the shaft of the key in the device of Kindler, and blocks the operating end of the key from being withdrawn from the keyhole. This is possible with keys of this type, since they are not modern keys, and the operating part of the key is substantially wider than the shaft. Consequently, a simple swinging lever retains the key.
The device of Helms is directed to the same type of key as used in the device of Kindler, but includes additional apparatus which is clamped to the shaft of the key to provide means for turning the key. These devices would not work with keys of the type currently used for deadbolt locks, since the keys currently used do not have an elongated shank on them, which is necessary for the devices of Kindler and Helms.
It is desirable to provide a lock conversion mechanism to retain the key in the key operated cylinder of a deadbolt lock, which can be used on conventional deadbolt locks without modifying the lock in any way, and which is simple in structure, simple to use, and simple to install.